This invention relates to mufflers of the sound-modifying type used with internal combustion engines to attenuate engine noise. More specifically, the invention relates to the construction of a muffler containing an insert having one or more means for directing the flow of exhaust gases and a plurality of reflecting surfaces for internal attenuation of sound waves.
Engine noise in an internal combustion engine typically is generated by the sudden expansion of combustion chamber exhaust gases. As the combustion gases are exhausted from each cylinder of the engine, a sound wave front travels at rapid sonic velocities through the exhaust system. This wave front is the boundary between the high pressure exhaust pulse and ambient pressure. When the sound wave front exits the exhaust system, it continues to pass through the air until three dimensional diffusion causes it to eventually dissipate. As the wave front passes an object, an overpressure is created at the surface of the object, and it is this overpressure that is the direct cause of audible and objectionable noise.
Since the inception of the internal combustion engine, efforts have been underway to reduce or muffle the noise caused by the engine. Obviously, considerable noise attenuation or reduction can be achieved in a muffler having dimensions that are large enough to permit three dimensional dissipation of the sound waves within the muffler housing. However, from a practical standpoint, design criteria often dictate that the size of the muffler must be kept as small as possible. Further means of attenuating engine noise include the use of packings and complex baffle systems. Often, these approaches are accompanied by a substantial increase in the back pressure or resistance of the muffler to the free discharge of the combustion gases. The increase in back pressure can result in a decrease in the output horsepower of the engine with a resulting loss of efficiency and fuel economy. Thus, it is clear that a goal of most muffler designs is to achieve maximum attenuation of sound within a muffler system of reasonable proportions while reducing or minimizing back pressure.
Increasingly, the public awareness and objection to environmental problems including "noise pollution" has led to more stringent noise control regulations. At the same time, there is a strong mandate for greater fuel economy and more complete fuel combustion along with smoother and more efficient operation of internal combustion engines. Achieving these seemingly incompatible objectives in a muffler design is a constant challenge.